Method for the manufacture of a waveguide mixer

ABSTRACT

A method is presented for writing a permanent spatially periodic phase-matching second-order non-linearity grating in an optical waveguide. Single frequency exciting radiation is launched in a pair of guided modes of the waveguide in the presence of an external DC electric field applied in a direction transverse to the waveguide. The exciting radiation may be either co-propagating or counter-propagating. The above method may be refined by performing it once for an estimated value of exciting radiation frequency, testing the waveguide to determine error in the chosen estimate, erasing the grating and re-performing the method using a corrected value for the frequency of the exciting radiation.

TECHNICAL FIELD

The present invention concerns improvements in or relating to methodsfor the manufacture of waveguide mixers. It concerns thus themanufacture of waveguides, in particular but not exclusively waveguidesimplemented using optical quality fibre. Mixers have application forsecond harmonic generation and for three wave mixing for frequency upconversion and or down conversion.

BACKGROUND

Frequency-mixing of optical waves has traditionally been achieved usingcrystals with non-inversion symmetric lattices. In the case ofthree-wave mixing, two waves of frequency ω₁, ω₂ are mixed to produceanother optical wave at either the sum or the difference frequency ω₃=ω₁ ±ω₂. The efficiency of the process is governed by two conditions:

(i) The crystal must have a high second-order non-linearity, whichinitiates the frequency-mixing process; and

(ii) The effective refractive indices (or propagation constants) of thethree waves must be matched in order to ensure that the waves propagatein-phase inside the crystal, a condition which is commonly referred toas phase-matching.

It has recently been shown that efficient frequency-mixing, inparticular second-harmonic generation (whereby a pump-wave at frequencyω is converted into a second-harmonic at frequency 2ω), may be obtainedin spatially-prepared optical fibre waveguides (Osterberg, U. andMargulis, W.: "Dye-laser pumped by Nd: Yag laser pulsesfrequency-doubled in a glass optical fibre", Opt. Lett., 1986, 11, p516; Farries, M. C. et al: "Second-harmonic generation in an opticalfibre by self-written-grating", Electron. Lett. 1987, 23, p 322; Stolen,R. H. and Tom, H. W. K: "Self-Organisation phase-matched harmonicgeneration in optical fibres", Opt. Lett., 1987, 12, p 585). The fibresare usually prepared by exciting them simultaneously with intenseradiation at two different wavelengths, a fundamental and the secondharmonic, e.g. 532 nm and 1.064 μm. This process has been shown toproduce a permanent spatially-periodic second-order susceptibility(χ.sup.(2)) in the fibre. The efficiency of of this process has been upto 10% for an input peak power of 1 kw (Farries M. C. "Efficientsecond-harmonic generation in an optical fibre", Proc. Colloquium onNon-Linear Optical Waveguides, London IEE 1988).

It is believed that the second-order susceptibility arises from theorientation of multi-photon-excited defect centres under the influenceof a self-induced internal dc-field. The internal field is generated bya third-order nonlinear process involving both the exciting radiationsat 1.064 μm and at 532 nm.

Recently, it has been shown that a much greater second-ordersusceptibility may be produced in a fibre by applying a large (>100V/μm) external dc electric-field across the fibre at the same time asdefects are being excited by intense blue light propagating in the corein a guided mode (Bergot, M. V. et al: "Generation of permanentoptically-induced second-order non-linearities in optical fibres bypoling"), Opt. Lett., 1988, 13, p. 592). The increase in x(²) is due tothe much larger electric field inside the fibre. However, in thisreported experiment, second-harmonic conversion efficiency was very low,since no phase-matching between applied infra-red wavelength waves wasachieved.

A degree of phase-matching has since been demonstrated using anon-periodic second-order non-linearity (Fermann, M. E. et al:"Frequency-doubling by modal phase-matching in poled optical fibres",Electron. Lett. 24, 1988, p. 894). Here phase-matching has been achievedby exploiting the phase velocity difference which occurs between thepump in the lowest-order guided mode and the second-harmonic in ahigher-order guided mode. This technique has the disadvantage that is isextremely sensitive to the small changes in fibre parameters along thelength.

DISCLOSURE OF INVENTION

This invention provides a technique for producing a periodic nonlinearsusceptibility in a waveguide which allows phase-matching forfrequency-mixing to be obtained between different guided modes insidethe waveguide. As a result efficient frequency-mixing may be obtained.The method is much less sensitive to fluctuations in waveguideparameters.

In accordance with the invention thus there is provided a method for themanufacture of a waveguide mixer, which method includes the followingprocedural steps; providing a waveguide which waveguide incorporates amultiplicity of excitable defect centres and is capable of sustaining aplurality of guided modes; launching an exciting radiation of apredetermined single frequency into at least two guided modes of thewaveguide; applying, simultaneously with propagation of the excitingradiation in said at least two guided modes, an external dc electricfield transverse to the waveguide thereby to produce in the waveguide apermanent spatially-periodic phase-matching second-order non-linearitygrating.

The waveguide specified above conveniently may have the form of anoptical fibre. This fibre may be of silica and include a core which isdoped. It is preferable that this core includes one or more of thefollowing dopants:

    GeO.sub.2 ; B.sub.2 O.sub.5 ; Al.sub.2 O.sub.3 ; or, B.sub.2 O.sub.3.

Mixer performance may be improved by pre-treatment of the providedwaveguide to enhance the concentration of the excitable defect centres.Such pre-treatment may include for example high energy radiation;exposure to hydrogen gas under elevated pressure; waveguide productionin oxygen starved ambient; or, addition of defect promoting dopants, forexample, alkali halides. In the case of a fibre waveguide, defectconcentration may also be enhanced by pulling at elevated temperature.

The exciting radiation launched in the guided modes of the waveguide maybe either co-propagating or counter-propagating.

It is possible to remove the aforesaid grating by bleaching. Thisproperty may be utilised in a refinement of the above method to achievehigher precision. In accordance with a further aspect of the inventionthus there is provided a method wherein the defined procedural steps arefirst conducted using exciting radiation of a predetermined estimatedfrequency followed by an applications trial at one or more designfrequencies to quantify error in said predetermined estimated frequency;erasing said permanent spatially-periodic phase-matching second-ordernon-linearity grating; and repeating said procedural steps usingexciting radiation of a predetermined corrected frequency.

The grating may be erased by treatment at an elevated temperature, oralternatively by launching exciting radiation into the waveguide in theabsence of any applied poling electric field.

BRIEF INTRODUCTION OF THE DRAWINGS

In the drawings accompanying the specification:

FIG. 1 is a schematic representation of a second-order susceptibilitygrating written by mode-interference between the fundamental and nexthigher-order mode in an optical fibre waveguide;

FIG. 2 is a graph showing beat lengths of modes in an optical fibre as afunction of wavelength;

FIG. 3 shows two cross-sections of different fibres with one or moreinternal electrodes; and

FIG. 4 is a graph showing measured second-harmonic conversion efficiencyas a function of pump-wavelength.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

So that this invention may be better understood embodiments thereforewill now be described and reference will be made to the drawingsaforesaid. The description that follows is given by way of example only.

For simplicity, in the text that follows, discussion of thefrequency-mixing process is restricted to the specific case ofsecond-harmonic generation.

Optical fibre poling involves the application of a strong dc-fieldacross a fibre and the simultaneous launching of high-intensity bluelight (the exciting radiation) into the fibre. This generates anon-periodic second-order susceptibility χ.sup.(2) inside the fibrecore, provided the fibre propagates only one single transverse mode (ora large number of transverse modes). In an intermediate regime, however,where the fibre sustains a few modes only, mode interference (or"beating") leads to a periodic blue-light intensity distribution insidethe fibre. Defects are then preferentially excited at the intensitymaxima and are subsequently aligned in the presence of the dc-field. Itis this intermediate regime that is utilised in the present method. Inthis way a second-order susceptibility grating may be written inside thefibre core.

That second-order susceptibility grating written by mode interferencebetween the LP₀₁ (E₁₁) and LP₁₁ (E₁₂) modes is shown schematically inFIG. 1. Here co-ordinate z represents the direction of propagation, andco-ordinates x and y the transverse co-ordinate of the fibre core(diameter d). The shaded regions represent regions of high opticalintensity in the core, the period Λ being the result of phase slippagebetween the two modes, which have different phase velocities. Theapplied poling electric-field E_(dc) is applied in the y-direction. Notethat because the interfering modes have different transverse fielddistributions, (i.e. the modes are of different order) the resultingintensity distribution varies transversely as well as longitudinally inthe core. The induced χ.sup.(2) - grating conforms to the power flowinside the fiber core, which follows a zig-zag line. In the figure, thearrows shown represent the aligned electric dipoles inside the fibrecore.

The grating period Λ due to modal interference is given by thedifference in propagation constants between the LP₀₁ (E₁₁) and the LP₁₁(E₁₂) modes at the blue writing-frequency ωL, i.e.: ##EQU1##

Where β (LP_(nm) ; ω_(b)) is the propagation constant of mode LP_(nm) atfrequency ω_(b). When high-intensity infra-red light is launched intothe fibre (the "reading" wave), phase-matching and thus efficientsecond-harmonic generation may be obtained for an infrared pump-wave atfrequency ω propagating in the LP₀₁ (E₁₁) mode and a second-harmonicwave at frequency 2ω propagating in a higher-order mode. Phase-matchingoccurs when the beat length Δβ corresponding to the difference in the ωand 2ω propagation constants equals the beat length of the second-ordersusceptibility grating previously written, i.e.: ##EQU2##

An example of an approximate choice of wavelengths for the blue("writing") modes and the infrared pump and second-harmonic ("reading")modes is shown in FIG. 2 for a typical fibre design, i.e. a standardgermanosilicate-core fibre with a numerical aperture NA= 0.30 and a coreradius of 1.3 μm. Illustrated is the grating period generated by a setof mode pairs at the blue writing wavelength of 488 nm and the beatlength corresponding to Δβ for an infrared pump-wave in the LP₀₁ (E₁₁)and its second-harmonic in the LP₁₁ (E₁₂) or LP₀₂ (E₁₃)-mode.Interference between the blue E₁₁ /E₁₂ -modes produces a χ.sup.(2)-grating having a 58 μm period. For the blue E₁₂ -E₁₃ -modes, the periodis 34 μm. Also displayed is the beat length corresponding to thedifference Δβ in propagation constants between the second-harmonic andpump-waves for an infrared pump-wave in the E₁₁ -mode and a SH in theE₁₂ -mode. Phase matching between the χ.sup.(2) -grating and thefundamental E₁₁ and SH E₁₂ -mode is obtained at those wavelengths forwhich the grating period equals Δβ, i.e. at 0.98 μm and 1.25 μm.

In the following, a mathematical description of the second-ordersusceptibility gratings written by mode-interference is given:

Without loss of generality, here it is assumed that blue light only islaunched into the fundamental LP₀₁ (E₁₁) and the second-order LP₁₁(E₁₂)-guided modes of the optical fibre. Further, it is assumed that thefibre is polarisation-preserving and that the guided modes (designated aand b) are linearly-polarised and plane-parallel to each other. Bothguided modes are considered to propagate equal; power. The normalisedfield distributions in the two modes are then given by:

    E.sub.a =ψ.sub.a (x,y)exp(iβ.sub.a z);            (1a)

and

    E.sub.b =ψ.sub.b (x,y)exp(iβ.sub.b z)             1b)

respectfully, where ψ_(a) ;_(b) are the transverse amplitudedistributions and β_(a;) b are their propagation constants. These guidedmodes interfere in the fibre to form an intensity distribution: ##EQU3##where G=β_(a) -β_(b) is the grating constant. If the guided modes are ofthe correct wavelength, i.e. around 480 nm for germanosilicate fibres,defect centres will be excited over a period of time, where thesteady-state defect excitation is approximately proportional to the bluelight intensity (Poyntz-Wright, L. J. et al: "Two-photon absorptionfibres", Opt. Lett., 13, 1988 p. 1023). If now a strong poling field isapplied along one of the optical axes of the fibre, a χ.sup.(2) 111tensor element parallel to the poling field is created, where, forsimplicity, it is assumed that in steady-state the induced.sub.χ.sup.(2) is proportional to the intensity of the blue lightinterference pattern. The χ.sup.(2) -distribution in the fibre core isthen given as:

    χ.sup.(2) (x,y,z)=χ.sub.o.sup.(2) (x,y)+χ.sub.m.sup.(2) (x,y)exp(e.sup.iGZ +e.sup.-iGZ);                          (3a)

where:

    χ.sub.o.sup.(2) =α[|ψ.sub.a |.sup.2 +|ψ.sub.b |.sup.2 ]; and            (3b)

    χ.sub.m.sup.(2) =αψ.sub.a ψb*            (3c)

are the transverse χ.sup.(2) -distributions corresponding to the zeroand first order Fourier components of χ.sup.(2) in spatial-frequencyspace and .sup.α is a constant. From the mixing of the LP₀₁ (E₁₁) andLP₁₁ (E₁₂) modes the distribution of χ.sup.(2) along the fibre is asshown schematically in FIG. 1. When an infrared pump-wave ψ_(p)(x,y)exp(iβ_(p) z) of frequency ω is launched into the fibre, theχ.sup.(2) -grating leads to coupling to a second-harmonic (SH)-wave:ψ_(sh) (x, y)exp(iβ_(sh) z) at 2ω. It may then be shown that the powergenerated in the SH-wave is given by (esu units): ##EQU4## wherep.sup.ω_(p) is the power in the pump-wave, χ.sup.(2)_(m) refers to theaverage over the effective core area A, n(ω) is the refractive index atfrequency ω, c is the velocity of light, Δβ=β_(sh) -2β_(p) and 0_(m) isthe overlap integral given by: ##EQU5##

In deriving equation (4) all phase terms for which G≠Δβ have beenneglected. It may be seen from equation (5) that the asymmetricχ.sup.(2) allows coupling of the LP₀₁ (pump-wave only to asymmetricSH-waves. In general, even when several blue modes interfere to form aχ.sup.(2) -grating, only pairs of modes can contribute to a χ.sup.(2) mat the spatial frequency given by their respective difference inpropagation constants. Therefore equation (5) effectively alwaysrepresents the amplitude overlap integral of five modes (two blue, twoinfrared and one SH), where in order to get SH-conversion, the productof their respective parities has to be positive.

A specific example will now be presented:

In order to align defect centres, the application of a very strongdc-electric field to the fibre core is necessary. DC-electric fields upto 200 V/μm may be applied to D-shaped fibres 1 (FIG. 3b) with aninternal electrode 3 and an external electrode 5. DC-electric fields upto 700 V/μm may be applied to fibres 9 with two internal electrodes 11and 13 (FIG. 3a). (By way of background reference, the reader isreferred to United Kingdom Patent Application No. 2,192,289 publishedJan. 6, 1988 which describes the manufacture of fibres includinginternal electrodes). In order to produce these types of fibre 1,9 astandard fibre preform is taken and either one or two holes are drillednext to the preform core 7. In order to produce a D-shaped preform, oneside of the preform 1 has to be removed by grinding. Fibre pulling isconventional although it is necessary to use relatively low pullingtemperatures in order to avoid a deformation of the fibre. The flat sideof a D-shaped fibre 1 does not have to be polished to give asufficiently smooth surface, since the pulling process smooths outunavoidable surface irregularities introduced by grinding. The internalelectrode 3 consists either of an InGa alloy with a melting temperatureof about 20° C. pumped into the holes in the fibre, or can beincorporated during the fibre-drawing process. In the case of theD-shaped fibre 1, the flat side is pressed onto a smooth metal platewhich provides the second electrode 5. Electrical contact is establishedby inserting a gold wire into the fibre 1 and soldering it onto acircuit board (not shown).

In the case of germanosilicate fibres, defect excitation using a cwArgon laser operating at 488 nm has so far produced the best results.The optimum excitation intensity is about 6 mW/μm² independent of polingfield strength; higher intensities lead to defect excitation saturationand a negative effect on SH-conversion efficiency. It is found that thesecond-order non-linear susceptibility χ.sup.(2) induced by poling islinearly proportional to the applied poling field strength. Using polingfield strengths of 600 V/μm, a χ.sup.(2) (2 ω=ω+ω) of about 30-90% ofthat of the well-known non-linear crystal potassium di-hydrogenphosphate KDP is inducable.

An example of second-harmonic generation (SHG) in the fibre describedearlier using phase matching provided by mode-interference gratings isshown in FIG. 4, which illustrates second-harmonic (SH) conversionefficiency as a function of pump wavelength. Here the grating waswritten by mode interference between the LP₀₂ (E₁₃) and the LP₁₁(E₁₂)-modes using cw defect-excitation light of 40 mW power at 488 nmand a poling field strength of 140 V/μm. SH-conversion is then obtainedfor an infrared pump-wave in the LP₀₁ (E₁₁)-mode and its SH in the LP₁₁(E₁₂)-mode. The amplitude overlap integral for this process is here only3%. The spatial coherence length is 6 cm. From FIG. 4 it will also benoted that there is a splitting for the respective phase-match peaks.This arises due to the two polarisation orientations possible for eachblue (writing) mode pair. Owing to fibre birefringence, two χ.sup.(2)-gratings of slightly different periods have been produced. Theseχ.sup.(2) -gratings have the same orientations.

From the figure, the SH-conversion efficiency is 1% for a pump-power of150 W at 1050 nm. Since the overlap integral could be increased tovalues >10% if the LP₀₁ (E₁₁) and LP₁₁ (E₁₂)-modes were employed in boththe writing and reading process and since a poling field strength fivetimes larger than used here is possible, conversion efficiencies oforder 10% with a pump power of only 10 W are predicted.

It is suggested that even higher non-linearities and conversionefficiencies could be achieved by co-doping the fibre with P₂ O₅.High-energy irradiation of the fibres prior to poling, hydrogentreatment, fibre fabrication under oxygen-starved conditions, dopingwith aluminium, or alkali halides (e.g. Na⁺), or fibre pulling at hightemperature is known to lead to increased defect concentrations andshould thus also lead to higher SH conversion efficiencies.

It has been found that mode-interference gratings are not disturbed bylow-intensity infrared light, i.e. intensities smaller than 10 W/μm². Onthe other hand, the gratings are bleachable by exposure to hightemperatures, i.e. >100° C. They are also bleachable by launching bluelight into a poled fibre without the presence of a poling field. Thislatter property provides the opportunity to write the gratings in atwo-stage process to obtain phase-matched SH-conversion at precisely thedesired infrared design wavelength (or wavelengths) as follows. Firstlya calculation of effective mode indices gives an approximate value ofthe blue writing wavelength required for obtaining SH-conversion at thedesign infrared wavelength (or wavelengths)-(owing to very small fibrenon-uniformities, an exact calculation is not possible). A first gratingis then written into the fibre and tested for the design infraredwavelength (or wavelengths). The grating is then erased. On the basis ofthis test measurement, in a second stage the blue writing wavelength canbe adjusted slightly (i.e. corrected) to obtain the desired designphase-match condition precisely.

Second-order susceptibility gratings may also be written bycounterpropagating blue waves of the same wavelength in the opticalfibre. For example, if the two counterpropagating waves are eachtravelling in a respective fundamental mode with propagation constants+β_(a) and -β_(a), a second-order susceptibility grating:

    χ.sup.(2) (x, y, z)=χO.sup.(2) (x, y)+χ.sub.m.sup.(2) (x, y) cos (2β.sub.a z),

is written in the fibre. This type of grating may then be used forfrequency mixing in a manner similar to that already described.

We claim:
 1. A method of manufacturing a waveguide mixer comprising thesteps of:providing a waveguide, which waveguide incorporates amultiplicity of excitable defect centres and is capable of sustaining aplurality of guided modes; launching through the waveguide an excitingradiation of a predetermined single frequency in at least two guidedmodes within the waveguide in the absence of an exciting radiation otherthan said single frequency; and applying simultaneously with propagationof the exciting radiation in said at least two guided modes, an externaldc electric field transverse to the waveguide, thereby to produce in thewaveguide a permanent spatially-periodic phase-matching second-ordernon-linearity grating.
 2. The method as claimed in claim 1 wherein thewaveguide is an optical fibre.
 3. The method as claimed in claim 2wherein the optical fibre is of silica and has a core doped with atleast one of the following dopants:

    GeO.sub.2 ; P.sub.2 O.sub.5 ; Al.sub.2 O.sub.3 ; and B.sub.2 O.sub.3.


4. The method as claimed in claim 1 further comprising the step ofpre-treating the waveguide to enhance concentration of the excitabledefect centres.
 5. The method claimed in claim 4 wherein thepre-treating step is performed by one of the following treatments: highenergy radiation; exposure to hydrogen gas under elevated pressure;production in oxygen starved ambient; and addition of a defect promotingdopant.
 6. The method as claimed in claim 2 further comprisingpre-treating the fibre by pulling at an elevated temperature to enhanceconcentration of excitable defect centres.
 7. The method as claimed inclaim 1 wherein the exciting radiation launched in said at least twodistinct guided modes is co-propagating.
 8. The method as claimed inclaim 1 wherein the exciting radiation launched in said at least twoguided modes is counter-propagating.
 9. The method as claimed in claim 1wherein the procedural steps are performed for an estimated value offrequency for the exciting radiation and followed by;test measurement todetermine an error in an estimated value of frequency of the excitingradiation; erasure of the grating; and repetition of the proceduralsteps performed using a corrected value for the frequency of theexciting radiation.
 10. The method as claimed in claim 9 wherein thegrating is erased by heat treatment at elevated temperature.
 11. Themethod as claimed in claim 9 wherein the grating is erased by launchingexciting radiation in the waveguide in the absence of applied externalelectric field.
 12. A method of manufacturing a waveguide mixercomprising the steps of:(A) providing a waveguide, which waveguideincorporates a multiplicity of excitable defect centres and is capableof sustaining a plurality of guided modes; (B) pre-treating thewaveguide to enhance concentration of the excitable defect centres bycarrying out one of the following treatments:(a) high energy radiation,(b) exposure to hydrogen gas under elevated pressure, (c) production inoxygen starved ambient, and (d) addition of a defect promoting dopant;(C) launching through the pretreated waveguide, an exciting radiation ofa predetermined single frequency in at least two guided modes within thewaveguide in the absence of an exciting radiation other than said singlefrequency; and (D) applying simultaneously with propagation of theexciting radiation in said at least two guided modes, an external dcelectric field transverse to the waveguide, thereby to produce in thewaveguide a permanent spatially-periodic phase-matching second-ordernon-linearity grating.
 13. The method as claimed in claim 12, whereinthe waveguide is a fibre, and wherein the pre-treating step is performedby pulling the fibre at an elevated temperature.